A number of different physiologic parameters are strong candidates for continuous monitoring, such as blood pressure or flow, cardiovascular pressure, intracranial pressure, intraocular pressure, glucose levels, etc. Wireless sensors in particular are highly desirable for biologic applications because the transcutaneous passage of wires (or other communication “tethers”) risks both infection to the patient and physical injury or device damage if the communication link experiences excessive pulling force.
A large number of proposed schemes for non-medical wireless communication rely on magnetic coupling between an electrical coil in an implanted device and a separate, external “readout” coil. One method of wireless communication well-known to those knowledgeable in the art is that of the LC tank resonator. A series-parallel connection of a capacitor and inductor has a specific resonant frequency ideally expressed as 1/√{square root over (LC)}, which can be measured from the impedance of the circuit. If one element of the inductor-capacitor pair varies with some physical parameter (e.g. pressure), while the other element remains at a known value, the physical parameter may be determined from the resonant frequency. For example, if the capacitor corresponds to a capacitive pressure sensor, the capacitance may be back-calculated from the resonant frequency. The sensed pressure may then be deduced from the capacitance by means of a calibrated pressure-capacitance transfer function.
The impedance of the LC tank may be measured directly or it may also be determined indirectly from the impedance of a separate readout coil that is magnetically coupled to the internal coil. The latter case is most useful for biologic applications in that the sensing device may be subcutaneously implanted, while the readout coil may be located external to the body, but in a location that allows magnetic coupling between the implant and readout coil. It is possible for the readout coil (or coils) to simultaneously excite the implant resonator and sense the impedance reflected back to the readout coil. Consequently, this architecture has the substantial advantage of requiring no internal power source, which greatly improves its prospects for long-term implantation (e.g. decades to a human lifetime).
Such devices have been proposed in various forms for many applications. Chubbuck (U.S. Pat. No. 4,026,276), Bullara (U.S. Pat. No. 4,127,110), and Dunphy (U.S. Pat. No. 3,958,558) disclose various devices initially intended for hydrocephalus applications (but also amenable to others) that use LC resonant circuits. The '276, '110, and '558 patents, although feasible, do not take advantage of recent advances in silicon (or similar) microfabrication technologies. Kensey (U.S. Pat. No. 6,015,386) discloses an implantable device for measuring blood pressure in a vessel of the wrist. This device must be “assembled” around the vessel being monitored such that it fully encompasses the vessel, which may not be feasible in many cases. In another application, Frenkel (U.S. Pat. No. 5,005,577) describes an implantable lens for monitoring intraocular pressure. Such a device would be advantageous for monitoring elevated eye pressures (as is usually the case for glaucoma patients); however, the requirement that the eye's crystalline lens be replaced will likely limit the general acceptance of this device.
In addition to the aforementioned applications that specify LC resonant circuits, other applications would also benefit greatly from such wireless sensing. Han, et al. (U.S. Pat. No. 6,268,161) describe a wireless implantable glucose (or other chemical) sensor that employs a pressure sensor as an intermediate transducer (in conjunction with a hydrogel) from the chemical into the electrical domain.
For example, the treatment of cardiovascular diseases such as Chronic Heart Failure (CHF) can be greatly improved through continuous and/or intermittent monitoring of various pressures and/or flows in the heart and associated vasculature. Porat (U.S. Pat. No. 6,277,078), Eigler (U.S. Pat. No. 6,328,699), and Carney (U.S. Pat. No. 5,368,040) each teach different modes of monitoring heart performance using wireless implantable sensors. In every case, however, what is described is a general scheme of monitoring the heart. The existence of a method to construct a sensor with sufficient size, long-term fidelity, stability, telemetry range, and biocompatibility is noticeably absent in each case, being instead simply assumed. Eigler, et al., come closest to describing a specific device structure although they disregard the baseline and sensitivity drift issues that must be addressed in a long-term implant. Applications for wireless sensors located in a stent (e.g., U.S. Pat. No. 6,053,873 by Govari) have also been taught, although little acknowledgment is made of the difficulty in fabricating a pressure sensor with telemetry means sufficiently small to incorporate into a stent.
Closed-loop drug delivery systems, such as that of Feingold (U.S. Pat. No. 4,871,351) have likewise been taught. As with others, Feingold overlooks the difficulty in fabricating sensors that meet the performance requirements needed for long-term implantation.
In nearly all of the aforementioned cases, the disclosed devices require a complex electromechanical assembly with many dissimilar materials, which will result in significant temperature- and aging-induced drift over time. Such assemblies may also be too large for many desirable applications, including intraocular pressure monitoring and/or pediatric applications. Finally, complex assembly processes will make such devices prohibitively expensive to manufacture for widespread use.
As an alternative to conventionally fabricated devices, microfabricated sensors have also been proposed. One such device is taught by Darrow (U.S. Pat. No. 6,201,980). Others are reported in the literature (see, e.g. Park, et al., Jpn. J. Appl. Phys., 37 (1998), pp. 7124-7128; Puers, et al., J. Micromech. Microeng. 10 (2000), pp. 124-129; Harpster et al., Proc. 14th IEEE Intl. Conf. Microelectromech. Sys. (2001), pp. 553-557).
Past efforts to develop wireless sensors have typically been limited to implant-readout separation distances of 1-2 cm, rendering them impractical for implantation much below the cutaneous layer. This would eliminate from consideration applications such as heart ventricle pressure monitoring or intracranial pressure monitoring, which require separation distances in the range of 5-10 cm. In the present state-of-the-art, several factors have contributed to this limitation, including: 1) signal attenuation due to intervening tissue, 2) suboptimal design for magnetic coupling efficiency; and 3) high internal energy losses in the implanted device.